Large structures, such as bridges, tunnels, tanks, ships, and so forth, are generally formed of concrete and steel. While these materials are strong and relatively inexpensive, corrosion and weathering results in a need to maintain, repair, or replace the structures. Replacement of structures is expensive and oftentimes inconvenient to the public. One technique for maintaining or repairing structures includes the use of abrasive blasting to clean or strip surfaces of the structure.
Abrasive blasting uses a variety of blast media, including sand, coal slag, garnet, steel grit, and other blast media, and is the most common method of removing corrosion and coatings from steel and concrete surfaces. In its simplest form, blast media under pressure is mixed with air in a metering valve directly under a blast pot or pressure vessel on an abrasive blasting machine. By controlling the amount of blast media that enters an airflow that leads out of a blast hose to a nozzle, an optimum balance of air and blast media can be achieved, thereby allowing for the highest productivity in preparing a surface, whether performed by automated equipment or human operators.
Industrial abrasive blasting processes, invented in the early 1900s, for use in preparing surfaces for protective coatings have always been relatively expensive based on a need for large volumes of high pressure compressed air at up to 200 psi. Equipment used to generate such pressures include the use of a stationary or mobile compressor (e.g., electric or diesel). Supporting the abrasive blasting process includes the use of blasting media, such as sand, steel grit, or other blast media to mix with the air, and labor to manage and operate nozzles used to control the blast media being directed onto the structures.
Abrasive blasting work to prepare surfaces for coating can be accomplished in a fixed facility, such as a blast room, where structural components to be treated are brought for the abrasive blasting process to be completed. Such fixed location processes are common where abrasive blasting is part of a manufacturing process that takes place before or after welding, but always before application of protective coatings. In mobile operations, where blasting equipment is moved to a structure needing abrasive blasting, such as is the case with preparation prior to coating of bridges, tanks, and ships, everything, including the blasting equipment and blast media, must be moved to the structure where the work is to take place. In the case of bridge work, this means being located out on a highway, where traffic is generally diverted during the abrasive blasting process. For storage tanks, the abrasive blasting equipment is moved to the location of the storage tank, be it water or petroleum, where the abrasive blasting work is to be done to the interior and/or exterior of the storage tank, as required. In shipyard work, the abrasive blasting equipment may be used in a dry dock, outside the dry dock, or placed some distance from the surfaces being cleaned. On a typical ship, the surface area of the internal ballast and fuel tanks is often 17 times as great as the actual surface of the hull of the vessel.